This invention relates generally to the field of acoustic monitoring for seismic or microseismic purposes. More specifically, the present invention is an acoustic receiver for sensing acoustic waves propagating through subterranean formations and generating triaxial acoustic response data representative of the waves.
In the production of hydrocarbons or the like from subterranean formations, it is common to hydraulically fracture a producing formation to increase the productivity from the formation. In a typical hydraulic fracturing operation, fluid is injected through the wellbore and into the formation at a high flow rate and at a pressure greater than the earth stress in the formation. This causes fractures to form in the formation, which fractures generally begin at the wellbore and radiate laterally away from the wellbore. It is desirable to know the length and direction (azimuth angle) of the fracture extending away from the well in order to predict with greater accuracy the influences of the fracture on the flow of fluids in the zone of interest. It is also important to determine the vertical extent of the hydraulic fracturing to determine if the fracture has grown to intersect other permeable zones above or below the zone of interest. As described further below, it is well known that an indication of hydraulic fracture direction, or azimuth angle, can be derived from microseismic events-also known as low energy acoustic emissions or waves in the earth-occurring when the hydraulic fracture is formed. impermeable barrier which bounds the intended injection zone can indicate such movement.
Microseismic events may also be produced in the subsurface by processes other than induced hydraulic fracturing of wells or pressure changes in a reservoir. For example, subsidence accompanying reservoir pressure reduction may lead to movement of piles or other equipment at the surface or seabed above a reservoir, producing additional microseismic events. Also, increase in pressure inside the casing of a well may cause mechanical failure of the cement sheath around the casing, and an acoustic wave may originate from very near the casing. If there is communication of fluid pressure along the wellbore outside the casing because of lack of a hydraulic seal by the cement, the pressure changes may cause microseismic events originating very near the casing. There may be a need to determine the location of these microseismic events or the magnitude of these events.
Furthermore, sources of acoustic waves in the subsurface are not limited to microseismic events. For example, a well flowing uncontrolled to the surface of the earth, called a xe2x80x9cblowoutxe2x80x9d, may flow at such high rates that significant acoustic noise is created at the bottom or at other segments of the well. There is often a need to locate the source of this noise in order to assist in attempts to stop the uncontrolled flow. Measurements of the source of the noise may be made from offset wells.
Wellbore acoustic receivers for detecting microseismic events or acoustic waves in a well have become widely available in recent years. An acoustic wave will travel away from the source with approximately spherical wave fronts consisting of a compressional xe2x80x9cPxe2x80x9d-wave phase and a shear xe2x80x9cSxe2x80x9d-wave phase, which can be used to determine the source location. Conventional acoustic receivers typically have three mutually orthogonal seismic sensors (geophones or accelerometers) for collecting three-component (i.e., x,y,z) data and include means for coupling the receivers to the casing of a well. The seismic signals received are transmitted to the surface of the earth by various means (e.g., conventional wireline) and are then processed to determine the seismic source location. Signals from acoustic receivers can be transmitted to the surface over wireline using frequency modulated telemetry signals. The multiple individual signals are recovered at the surface by bandpass filtering and converted to amplitude modulated signals. Alternatively, signals from the receivers may be digitized downhole and transmitted to the surface in real-time over a fiber optic cable or copper wire.
However, it may be difficult to obtain useable data from conventional acoustic receivers because microseismic events generated by induced hydraulic fractures or arising from other sources vary widely in amplitude. The amplitudes of events in the range of interest for microseismic monitoring may vary by four or more orders of magnitude as a result of source variability and attenuation due to source receiver separation. The dynamic range of the signals may exceed the dynamic range of the acoustic receiver. In addition to this signal amplitude problem, there are geometric considerations related to sensor placement which cause additional variability in the apparent signal strength of an acoustic emission detected by conventional acoustic receivers. The inventive device is designed to mitigate these geometry effects on signal reception.
Most acoustic receivers used for vertical seismic profiling tools consist of an orthogonal triaxial set of sensors, often geophones, with one sensor oriented along the wellbore axis and two orthogonal sensors in the orthogonal plane to record the full three-component (x-y-z) response vector. The strength of the acoustic sensor impulse reading will depend on the amplitude of the seismic event as well as the position of the seismic event relative to the acoustic receiver. If the angle between the axis of an acoustic sensor and the impinging acoustic energy is small, then the full event amplitude is applied to that acoustic sensor. However, if the angle is large, perhaps nearly 90xc2x0, then the applied signal strength along the axis of this sensor is significantly reduced. This amplitude level may be below the linear acoustic response threshold of the sensor. This geometric factor further adds to the difficult dynamic range problem that exists as a result of the source magnitude variability.
Accordingly, there is a need for an improved acoustic receiver design that has a more balanced response, without full dependence for waveform recording on a sensor that may be nearly orthogonal to the impinging acoustic wave motion. This would allow the response of the acoustic receiver to be subject to only the dynamic range variability of the seismic source without dependence on the geometric factors associated with position of the seismic event relative to the acoustic sensor.
The present invention is an acoustic receiver for sensing acoustic waves and generating orthogonal triaxial acoustic response data representative of the acoustic waves. The acoustic receiver of the present invention comprises at least four acoustic sensors oriented in at least four different directions. Each of the acoustic sensors is adapted to produce an electrical signal in response to an acoustic wave impinging on the sensors. The acoustic receiver further comprises a means for combining the electrical signals to yield orthogonal triaxial acoustic response data. In one embodiment, the acoustic receiver comprises at least five acoustic sensors oriented in at least five different directions: The acoustic sensors are wired in series pairs to produce a combined output along two of the three orthogonal axes, and one acoustic sensor is oriented substantially parallel to the longitudinal axis of the acoustic receiver. In another embodiment, at least one of the acoustic sensors is oriented substantially parallel to the longitudinal axis of the acoustic receiver and four of the acoustic sensors are angularly oriented at approximately 0xc2x0, 45xc2x0, 90xc2x0, and 135xc2x0, respectively, from a designated reference position. A method for sensing acoustic waves and generating orthogonal triaxial acoustic response data representative of the acoustic waves is also disclosed. The method comprises providing at least four acoustic sensors oriented in at least four different directions, with each of the acoustic sensors adapted to produce an electrical signal in response to an acoustic wave impinging thereon and combining said electrical signals to yield orthogonal triaxial acoustic response data.